Pipettes are used to dispense known volumes of liquid. Typically, a vacuum is applied to the mouth piece end of the pipette to draw a volume of liquid into the lumen of the pipette from a liquid reservoir, such as a bottle. Portions of the volume of liquid are then dispensed to one or more other containers. In many instances, exact volumes liquid must be accurately dispensed. To this end, many pipettes include graduations that indicate the volume of liquid in the lumen of the pipette, e.g., 1 milliliter (ml), 0.1 ml, etc. The internal diameter of the pipette determines the volume of the lumen at a given length of the pipette. The accuracy of the graduations is likewise determined at least in part by the internal diameter of the lumen of the pipette. Pipettes having a smaller internal diameter may more accurately dispense smaller volumes of liquid than pipettes with a larger internal diameter because the smaller volume is spread over a greater length of the pipette. However, the total volume of liquid capable of being dispensed by a pipette with a small internal diameter generally is limited by the practical length of the pipette. The practical length of a pipette is typically limited by the ability of the user to operate the pipette. For example, many pipettes are operated in a ventilated hood environment having a limited workspace area. Pipettes over a certain length are not practical for use in such an environment. Thus, highly accurate pipettes, such as those used to accurately dispense fractions of a milliliter, have a small internal diameter with a very limited volume.
In contrast to the exacting requirements for accurately dispensing very small volumes, in some uses the dispensed volume is not required to be as accurately dispensed, such as when dispensing multiple volumes of 1 ml or more. In these circumstances, a less accurate pipette with a larger internal diameter may be used.
In response to the various needs of different users, many manufacturers produce pipettes in a variety of maximum volume capacities, such as 1 ml, 2 ml, 5 ml, 10 ml, 25 ml, 50 ml, and 100 ml volumes. Larger volume pipettes sacrifice dispensing accuracy for increased volume. Likewise, highly accurate dispensing pipettes sacrifice volume for accuracy. Most laboratories have a mixed need for highly accurate and high volume pipettes. As such, these laboratories will typically stock a variety of different pipettes to meet its needs. The need to stock multiple sizes of pipettes can present stocking problems for the laboratory.
Perlman, U.S. Pat. No. 4,877,585, attempted to solve this problem by providing a graduated pipette with a generally cylindrical upper tube capable of delivering large volumes joined to a generally cylindrical lower tube with a smaller internal diameter for delivering small volumes. The upper and lower tubes are separately formed and then joined together with a cylindrical connector or by being welded together. Tubes that are joined together in these fashions may fail at the site of the connector or weld causing leakage, or even more detrimentally, causing the lower tube to fall way from the upper tube. In addition, the generally cylindrical upper and lower tubes each deliver a constant volume of liquid per unit of length of the respective tube. Thus, outside of the transition between the two tubes, the accuracy of the graduated markings on the individual tubes remains constant. A need for a unitary pipette with increased dispensing accuracy near the tip of the pipette compared to the dispensing accuracy nearer the mouthpiece was identified.
One method of making pipettes utilizes injection molding. However, injection molding processes inject thermoplastic materials under very high pressures into the mold. The high pressure injection of thermoplastic materials imparts significant forces at the injections site on the core used to form the lumen of the pipette during the injection process. As discussed above, highly accurate pipettes have a lumen with a relatively small internal diameter. Accordingly, the core of the injection mold necessarily has a relatively small external diameter. The high pressure imparted on the core can result in deflection of the core during the injection process. Core deflection can decrease the accuracy of the resulting pipette. Fay et al., U.S. Pat. No. 5,240,397 addressed the core deflection problem with a complicated injection molding process that used paired retractable locking pins for stabilizing the core during injection, solenoid drives for extending and contracting the pins, and a controller for retracting the pins as the injected resin flows along the core to the space proximate the pins. A need for a simple method of producing a highly accurate injection molded pipette to correct the deflection problem was identified.
In addition to concerns relating to the deflection of the thin core needed for highly accurate pipettes, there is also a risk of core breakage during removal of the pipette body. As the injected resin cools, it shrinks onto the core. Long thin core pins use to generate highly accurate pipettes are subject to locking forces as the resin cools. When the cooling pipette is removed from the core, these locking forces can cause the long thin cores to break. A need for a method of producing a highly accurate injection molded pipette with a decreased risk of breaking the core was identified.